Epitaxial wafer and method for manufacturing same

ABSTRACT

An epitaxial wafer includes a silicon carbide film having a first main surface. A groove portion is formed in the first main surface. The groove portion extends in one direction along the first main surface. Moreover, a width of the groove portion in the one direction is twice or more as large as a width of the groove portion in a direction perpendicular to the one direction. Moreover, a maximum depth of the groove portion from the first main surface is not more than 10 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/108,402, filed Jun. 27, 2016, which is a 371 application ofInternational Application No. PCT/JP2015/070844, filed Jul. 22, 2015,which claims the benefit of Japanese Patent Application No. 2014-157717,filed Aug. 1, 2014.

TECHNICAL FIELD

The present disclosure relates to an epitaxial wafer and a method formanufacturing the epitaxial wafer.

BACKGROUND ART

Japanese Patent Laying-Open No. 2014-17439 (Patent Document 1) disclosesa semiconductor manufacturing device used to manufacture an epitaxialwafer.

CITATION LIST Patent Document

PTD 1: Japanese Patent Laying-Open No. 2014-17439

SUMMARY OF INVENTION

An epitaxial wafer according to the present disclosure includes asilicon carbide film having a first main surface. A groove portion isformed in the first main surface of the silicon carbide film. The grooveportion extends in one direction along the first main surface. Moreover,a width of the groove portion in the one direction is twice or more aslarge as a width of the groove portion in a direction perpendicular tothe one direction. Moreover, a maximum depth of the groove portion fromthe first main surface is not more than 10 nm.

A method for manufacturing an epitaxial wafer according to the presentdisclosure includes the steps of: preparing a silicon carbide substratehaving a second main surface; and epitaxially growing a silicon carbidefilm on the second main surface. The step of epitaxially growing thesilicon carbide film includes the steps of: epitaxially growing a firstfilm on the second main surface using a source material gas having aC/Si ratio of less than 1; reconstructing a surface of the first filmusing a mixed gas including (i) a source material gas having a C/Siratio of less than 1 and (ii) a hydrogen gas; and epitaxially growing asecond film on the reconstructed surface of the first film using asource material gas having a C/Si ratio of not less than 1.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view showing a portion of anepitaxial wafer of the present disclosure.

FIG. 2 is a schematic plan view showing a portion of the epitaxial waferof the present disclosure.

FIG. 3 is a schematic plan view showing a portion of the epitaxial waferof the present disclosure.

FIG. 4 is a flowchart schematically showing a method for manufacturingthe epitaxial wafer of the present disclosure.

FIG. 5 is a schematic view showing a configuration of an epitaxialgrowth device.

FIG. 6 is a schematic view showing a cross section taken along a linesegment VI-VI in FIG. 5.

DESCRIPTION OF EMBODIMENTS Description of Embodiments of PresentDisclosure

First, embodiments of the present disclosure are listed and described.

[1] An epitaxial wafer 100 according to the present disclosure includesa silicon carbide film 120 having a first main surface 101. A grooveportion 20 is formed in first main surface 101, groove portion 20extending in one direction along first main surface 101, a width ofgroove portion 20 in the one direction being twice or more as large as awidth of groove portion 20 in a direction perpendicular to the onedirection, a maximum depth of groove portion 20 from first main surface101 being not more than 10 nm.

Hereinafter, the width of groove portion 20 in the one direction will bereferred to as “second width 82”, the width of groove portion 20 in thedirection perpendicular to the one direction will be referred to as“third width 83”, and the maximum depth of groove portion 20 from firstmain surface 101 will be referred to as “second depth 72”.

When epitaxially growing a silicon carbide film on a silicon carbidesubstrate, minute pit portions may be formed in a surface of the siliconcarbide film. Each of such pit portions is formed due to a threadingdislocation transferred from the silicon carbide substrate to thesilicon carbide film, and is a depression having a depth of aboutseveral ten nm. The present inventor has found that these pit portionscause increase of variation in film thickness of an oxide film formed onthe surface of the silicon carbide film and the variation in filmthickness is one factor for decrease of long-term reliability of asilicon carbide semiconductor device.

The present inventor has found that the formation of pit portions can besuppressed under a specific epitaxial growth condition. According to thegrowth condition, the pit portions are reduced whereas a multiplicity ofgroove portions are formed which are shallower than the pit portions andwhich extend in one direction. However, these groove portions areshallower than the pit portions and therefore have a smaller influenceover the film thickness of the oxide film than the influence of the pitportions.

In epitaxial wafer 100 described above, groove portion 20 is formed infirst main surface 101 of silicon carbide film 120 to extend in onedirection such that a ratio of second width 82 to third width 83 is notless than 2 and to have second depth 72 of not more than 10 nm. That is,by controlling the conditions for epitaxial growth of silicon carbidefilm 120 and the like, epitaxial wafer 100 is provided with a largernumber of groove portions 20 than the above-described pit portions eachhaving a depth of several ten nm. Hence, according to epitaxial wafer100, variation in film thickness of the oxide film can be reduced ascompared with the conventional epitaxial wafer in which the multiplicityof pit portions are formed.

The shape of the “groove portion” can be specified by observing firstmain surface 101 using a predetermined defect inspection device.Accordingly, second width 82 and third width 83 of groove portion 20 canbe measured. As the defect inspection device, WASAVI series “SICA 6×”provided by Lasertec Corporation (objective lens: ×10) can be used, forexample. Moreover, the depth of the “groove portion” can be measuredusing an AFM (Atomic Force Microscope).

[2] In epitaxial wafer 100 described above, groove portion 20 mayinclude a first groove portion 21 and a second groove portion 22connected to first groove portion 21. First groove portion 21 may beformed in one end portion of groove portion 20 in the one direction.Second groove portion 22 may extend in the one direction from firstgroove portion 21 to the other end portion opposite to the one endportion, and first depth 71, which is a depth of second groove portion22 from first main surface 101, may be smaller than second depth 72,which is the maximum depth of first groove portion 21.

In epitaxial wafer 100 in which groove portion 20 having the abovestructure is formed, the formation of pit portions that would haveotherwise caused increase of variation in film thickness of the oxidefilm is suppressed. Accordingly, according to epitaxial wafer 100,variation in film thickness of the oxide film can be reduced.

[3] Epitaxial wafer 100 may further include a silicon carbide substrate110 having a second main surface 102 having an off angle of not morethan ±4° relative to a (0001) plane. Silicon carbide film 120 may be asilicon carbide single crystal film formed on second main surface 102,and groove portion 20 may be formed to extend from a threadingdislocation 40 in silicon carbide film 120 in a step-flow growthdirection that is along an off direction of the off angle.

As described above, groove portion 20 may be formed to extend in thestep-flow growth direction. In epitaxial wafer 100 in which such agroove portion 20 is formed, the formation of minute pits that wouldhave otherwise caused decrease of long-term reliability of the siliconcarbide semiconductor device is suppressed. Accordingly, according toepitaxial wafer 100, variation in film thickness of the oxide film canbe reduced.

[4] In epitaxial wafer 100, the off direction may be in a range of notmore than ±5° relative to a <11-20> direction. Thus, second main surface102 may be inclined relative to a (0001) plane in the predetermined offdirection.

[5] In epitaxial wafer 100, the off direction may be in a range of notmore than ±5° relative to a <01-10> direction. Thus, second main surface102 may be inclined relative to the (0001) plane in the predeterminedoff direction.

[6] A method for manufacturing an epitaxial wafer according to thepresent disclosure includes the steps of: preparing (S10) a siliconcarbide substrate 110 having a second main surface 102; and epitaxiallygrowing (S20) a silicon carbide film on the second main surface. Thestep of epitaxially growing the silicon carbide film includes the stepsof: epitaxially growing a first film 121 on second main surface 102using a source material gas having a C/Si ratio of less than 1;reconstructing a surface of first film 121 using a mixed gas including(i) a source material gas having a C/Si ratio of less than 1 and (ii) ahydrogen gas; and epitaxially growing a second film 122 on thereconstructed surface of first film 121 using a source material gashaving a C/Si ratio of not less than 1.

In [6] described above, the “C/Si ratio” represents a ratio of thenumber of carbon (C) atoms to the number of silicon (Si) atoms in thesource material gas. The expression “reconstructing the surface”indicates to change a surface property of the first film through etchingby the hydrogen gas and through epitaxial growth by the source materialgas. Through the step of reconstructing, the thickness of the first filmmay be decreased, may be increased, or may be substantially unchanged.

In the step of reconstructing the surface, the ratio of the flow rate ofthe source material gas to the flow rate of the hydrogen gas may bereduced as compared with general epitaxial growth such that the etchingby the hydrogen gas is comparable to the epitaxial growth by the sourcematerial gas. For example, it is considered to adjust the flow rate ofthe hydrogen gas and the flow rate of the source material gas to attaina film formation rate of about 0±0.5 μm/h.

The above-described threading dislocations include threading screwdislocations, threading edge dislocations, and composite dislocations inwhich these dislocations are mixed. These dislocations are expressed byBurgers vector b in the following manner: threading screw dislocations(b=<0001>); threading edge dislocations (b=⅓<11-20>); and compositedislocations (b=<0001>+⅓<11-20>). It is considered that the pit portionshaving an influence over variation in film thickness of the oxide filmare formed due to the threading screw dislocations, the threading edgedislocations, and the composite dislocations. Pit portions formed due tothe threading screw dislocations and composite dislocations bothinvolving relatively large strain around the dislocations have deepdepths.

In [6] described above, the surface of the first film is reconstructed,whereby it can be expected to obtain an effect of attaining shallow pitportions formed due to threading screw dislocations and compositedislocations. In addition to this, the C/Si ratio of the source materialgas is changed from a value of less than 1 to a value of not less than 1and the second film is then grown. Accordingly, it is considered toincrease the effect of attaining shallow pit portions resulting fromthreading screw dislocations and composite dislocations.

Details of Embodiments of Present Disclosure

Next, with reference to figures, a specific example of one embodiment(hereinafter, referred to as “the present embodiment”) of the presentdisclosure will be described. It should be noted that in thebelow-described figures, the same or corresponding portions are giventhe same reference characters and are not described repeatedly. Further,in the present specification, an individual orientation is representedby [ ], a group orientation is represented by < >, and an individualplane is represented by ( ), and a group plane is represented by { }. Inaddition, a negative index is supposed to be crystallographicallyindicated by putting “−” (bar) above a numeral, but is indicated byputting the negative sign before the numeral in the presentspecification.

<Structure of Epitaxial Wafer>

First, the following describes a configuration of an epitaxial waferaccording to the present embodiment with reference to FIG. 1 to FIG. 3.FIG. 1 partially shows a cross sectional structure of the epitaxialwafer according to the present embodiment. Each of FIG. 2 and FIG. 3partially shows a planar structure of the epitaxial wafer according tothe present embodiment. FIG. 1 shows a cross sectional structure along aline segment I-I shown in FIG. 2 and FIG. 3.

As shown in FIG. 1, epitaxial wafer 100 according to the presentembodiment has a silicon carbide substrate 110 and a silicon carbidefilm 120. Silicon carbide substrate 110 is composed of a silicon carbidesingle crystal, for example. This silicon carbide single crystal has ahexagonal crystal structure and has a polytype of 4H, for example.Silicon carbide substrate 110 includes an n type impurity such asnitrogen (N) and therefore has n type conductivity. Silicon carbidesubstrate 110 has a diameter of not less than 100 mm (not less than 4inches), preferably, not less than 150 mm (not less than 6 inches), forexample.

Silicon carbide substrate 110 has a second main surface 102 and a thirdmain surface 103 opposite to second main surface 102. As shown in FIG.1, second main surface 102 is a main surface on which silicon carbidefilm 120 is formed. Second main surface 102 has an off angle of not morethan ±4° relative to a (0001) plane (hereinafter, referred to as“silicon (Si) plane”). The off direction of this off angle may be in arange of not more than ±5° relative to a <11-20> direction or may be ina range of not more than ±5° relative to a <01-10> direction, forexample.

Silicon carbide film 120 is a silicon carbide single crystal film formedon second main surface 102 by vapor phase epitaxy such as CVD. Morespecifically, silicon carbide film 120 is an epitaxial growth layerformed by CVD employing silane (SiH₄) and propane (C₃H₈) as a sourcematerial gas and nitrogen (N₂) or ammonia (NH₃) as a dopant gas.Moreover, silicon carbide film 120 includes nitrogen (N) atoms, whichare generated through thermal decomposition of the nitrogen or ammonia,and therefore has n type conductivity type. The n type impurityconcentration of silicon carbide film 120 is lower than the n typeimpurity concentration of silicon carbide substrate 110. It should benoted that since second main surface 102 is angled off relative to the(0001) plane as described above, silicon carbide film 120 is formedthrough step-flow growth. Hence, silicon carbide film 120 is composed of4H type silicon carbide as with silicon carbide substrate 110 andtherefore a different type of polytype is suppressed from being mixedtherein. Silicon carbide film 120 has a thickness of about not less than10 μm and not more than 50 μm, for example.

A groove portion 20 is formed in a surface of silicon carbide film 120,i.e., a first main surface 101. As shown in FIG. 2, groove portion 20extends in one direction along first main surface 101 in a plan view offirst main surface 101. More specifically, groove portion 20 extends ina step-flow growth direction, which is along the off direction of theoff angle of first main surface 101 relative to the (0001) plane. Inother words, groove portion 20 extends in a direction in a range of notmore than ±5° relative to the <11-20> direction or a direction in arange of not more than ±5° relative to the <01-10> direction.

It should be noted that FIG. 1 to FIG. 3 are drawn such that the“step-flow growth direction” corresponds to the X-axis direction in FIG.1 to FIG. 3. In each of FIG. 1 to FIG. 3, the X-axis direction, Y-axisdirection, and Z-axis direction are orthogonal to one another. TheY-axis direction shown in each of FIG. 2 and FIG. 3 represents adirection perpendicular to the step-flow growth direction. The Z-axisdirection shown in FIG. 1 represents the thickness direction of thesilicon carbide film.

The width (second width 82) of groove portion 20 in the above-describedone direction is twice or more as large as, preferably, five times ormore as large as the width (third width 83) thereof in the directionperpendicular to the one direction. Second width 82 is not less than 15μm and not more than 50 μm, preferably, not less than 25 μm and not morethan 35 μm. Moreover, third width 83 is not less than 1 μm and not morethan 5 μm, preferably, not less than 2 μm and not more than 3 μm.

As shown in FIG. 1, groove portion 20 is formed to extend in thestep-flow growth direction from a threading dislocation 40 included insilicon carbide film 120. More specifically, groove portion 20 includes:a first groove portion 21 formed on threading dislocation 40; and asecond groove portion 22 formed to be connected to first groove portion21 and extend from first groove portion 21 in the step-flow growthdirection.

First groove portion 21 is formed at one end portion (left end portionin FIG. 1) of groove portion 20 in the step-flow growth direction.Moreover, the maximum depth (second depth) of first groove portion 21from first main surface 101 is not more than 10 nm. Second depth 72 isthe maximum depth in the entire groove portion 20 as shown in FIG. 1.Moreover, first groove portion 21 preferably has a width (first width81) of not more than 1 μm, and more preferably has a width (first width81) of not more than 0.5 μm.

As shown in FIG. 1, second groove portion 22 is formed to extend fromits portion of connection with first groove portion 21 to the other endportion opposite to the above-described one end portion (right endportion in FIG. 1). Moreover, second groove portion 22 is formed suchthat a depth (first depth 71) of second groove portion 22 from firstmain surface 101 is smaller than the maximum depth (second depth 72) offirst groove portion 21. More specifically, second groove portion 22extends in the step-flow growth direction while maintaining the depthshallower than second depth 72. First depth 71 is preferably not morethan 3 nm, is more preferably not more than 2 nm, and is furtherpreferably not more than 1 nm. Moreover, second groove portion 22 has awidth (fourth width 84) of, for example, not less than 20 μm,preferably, not less than 25 μm.

<Method for Manufacturing Epitaxial Wafer>

Next, the following describes a method for manufacturing the epitaxialwafer according to the present embodiment. As shown in FIG. 4, themanufacturing method includes: a step (S10) of preparing a siliconcarbide substrate; and a step (S20) of epitaxially growing a siliconcarbide film.

First, as the step (S10), a step of preparing the silicon carbidesubstrate is performed. In this step (S10), a 4H type silicon carbideingot (not shown) obtained through crystal growth using, for example, asublimation-recrystallization method is sliced into a predeterminedthickness, thereby preparing silicon carbide substrate 110 (FIG. 1)having second main surface 102 and third main surface 103.

Next, as the step (S20), a step of growing the silicon carbide film isperformed. In this step (S20), as shown in FIG. 1, silicon carbide film120 is epitaxially grown using CVD on second main surface 102 of siliconcarbide substrate 110. First, the following describes a configuration ofan epitaxial growth device 1 used in this step (S20). FIG. 5 is a sideview of epitaxial growth device 1. FIG. 6 is a cross sectional view ofepitaxial growth device 1 along a line segment VI-VI in FIG. 5.

As shown in FIG. 5 and FIG. 6, epitaxial growth device 1 mainly includesheating elements 6, a heat insulator 5, a quartz tube 4, and aninduction heating coil 3. Each of heating elements 6 is composed of acarbon material, for example. As shown in FIG. 6, heating element 6 hasa semi-cylindrical hollow structure including a curved portion 7 and aflat portion 8. Two heating elements 6 are provided and disposed suchthat their respective flat portions 8 face each other. A spacesurrounded by these flat portions 8 is a channel 2 serving as a spacefor performing a treatment to silicon carbide substrate 110.

Heat insulator 5 is a member configured to thermally insulate channel 2from the outside of epitaxial growth device 1. Heat insulator 5 isprovided to surround the outer circumference portions of heatingelements 6. Quartz tube 4 is provided to surround the outercircumference portion of heat insulator 5. Induction heating coil 3 iswound at the outer circumference portion of quartz tube 4.

Next, the following describes a crystal growth process employingepitaxial growth device 1 described above. First, silicon carbidesubstrate 110 prepared in the step (S10) is placed in channel 2 ofepitaxial growth device 1. More specifically, silicon carbide substrate110 is placed on a susceptor (not shown) provided on one heating element6.

1. Step (S21) of Epitaxially Growing First Film

Next, a step of epitaxially growing a first film is performed. In thisstep, a source material gas having a C/Si ratio of less than 1 is usedto epitaxially grow a first film 121 (see FIG. 1) on second main surface102 of silicon carbide substrate 110. First, after gas replacement inchannel 2, a pressure in channel 2 is adjusted to a predeterminedpressure such as 60 mbar to 100 mbar (6 kPa to 10 kPa) while letting acarrier gas to flow. The carrier gas may be, for example, hydrogen (H₂)gas, argon (Ar) gas, helium (He) gas, or the like. The flow rate of thecarrier gas may be about 50 slm to 200 slm, for example. The unit forflow rate as used herein, i.e., “slm (Standard Liter per Minute)”represents “L/min” in a standard condition (0° C. and 101.3 kPa).

Next, a predetermined alternating current is supplied to the inductionheating coil, thereby inductively heating heating elements 6.Accordingly, channel 2 and the susceptor having silicon carbidesubstrate 110 placed thereon are heated to a predetermined reactiontemperature. On this occasion, the susceptor is heated to about 1500° C.to 1750° C., for example.

Next, a source material gas is supplied. The source material gasincludes a Si source gas and a C source gas. Examples of the Si sourcegas includes silane (SiH₄) gas, disilane (Si₂H₆) gas, dichlorosilane(SiH₂Cl₂) gas, trichlorosilane (SiHCl₃) gas, silicon tetrachloride(SiCl₄) gas, and the like. That is, the Si source gas may be at leastone selected from a group consisting of silane gas, disilane gas,dichlorosilane gas, trichlorosilane gas and silicon tetrachloride gas.

Examples of the C source gas includes methane (CH₄) gas, ethane (C₂H₆)gas, propane (C₃H₈) gas, acetylene (C₂H₂) gas, and the like. That is,the C source gas may be at least one selected from a group consisting ofmethane gas, ethane gas, propane gas, and acetylene gas.

The source material gas may include a dopant gas. Examples of the dopantgas include nitrogen gas, ammonia gas, and the like.

The source material gas in the step of epitaxially growing the firstfilm may be a mixed gas of silane gas and propane gas, for example. Inthe step of epitaxially growing the first film, the C/Si ratio of thesource material gas is adjusted to less than 1. For example, the C/Siratio may be not less than 0.5, not less than 0.6, or not less than 0.7as long as the C/Si ratio is less than 1. Moreover, for example, theC/Si ratio may be not more than 0.95, not more than 0.9, or not morethan 0.8. The flow rate of the silane gas and the flow rate of thepropane gas may be adjusted appropriately in a range of about 10 to 100sccm to achieve a desired C/Si ratio, for example. The unit for flowrate as used herein, i.e., “sccm (Standard Cubic Centimeter per Minute)”represents “mL/min” in a standard condition (0° C. and 101.3 kPa).

A film formation rate in the step of epitaxially growing the first filmmay be about not less than 3 μm/h and not more than 30 μm/h, forexample. The first film has a thickness of not less than 0.1 μm and notmore than 150 μm, for example. The thickness of the first film may benot less than 0.2 μm, may be not less than 1 μm, may be not less than 10μm, or may be not less than 15 μm. Moreover, the thickness of the firstfilm may be not more than 100 μm, may be not more than 75 μm, or may benot more than 50 μm.

2. Step (S22) of Reconstructing Surface of First Film

Next, a step of reconstructing a surface of the first film is performed.The step of reconstructing the surface may be performed continuous tothe step of epitaxially growing the first film. Alternatively, apredetermined halt time may be provided between the step of epitaxiallygrowing the first film and the step of reconstructing the surface. Inthe step of reconstructing the surface, the temperature of the susceptormay be increased by about 10° C. to 30° C.

In the step of reconstructing the surface, a mixed gas including asource material gas having a C/Si ratio of less than 1 and hydrogen gasis used. The C/Si ratio of the source material gas may be lower than theC/Si ratio in the step of epitaxially growing the first film. The C/Siratio may be not less than 0.5, not less than 0.6, or not less than 0.7as long as the C/Si ratio is less than 1. Moreover, for example, theC/Si ratio may be not more than 0.95, not more than 0.9, or not morethan 0.8.

In the step of reconstructing the surface, there may be used a sourcematerial gas different from the source material gas used in each of thestep of epitaxially growing the first film and a below-described step ofepitaxially growing a second film. In this way, it is expected toincrease an effect of attaining a shallow pit portion. For example, itis considered to configure such that in each of the step of epitaxiallygrowing the first film and the below-described step of epitaxiallygrowing the second film, silane gas and propane gas are used, whereas inthe step of reconstructing the surface, dichlorosilane and acetylene areused.

In the step of reconstructing the surface, the ratio of the flow rate ofthe source material gas to the flow rate of the hydrogen gas may bedecreased as compared with those in the step of epitaxially growing thefirst film and the below-described step of epitaxially growing thesecond film. Accordingly, it is expected to increase the effect ofattaining a shallow pit portion.

The flow rate of the hydrogen gas in the mixed gas is about not lessthan 100 slm and not more than 150 slm, for example. The flow rate ofthe hydrogen gas may be about 120 slm, for example. The flow rate of theSi source gas in the mixed gas may be not less than 1 sccm and not morethan 5 sccm, for example. The lower limit of the flow rate of the Sisource gas may be 2 sccm. The upper limit of the flow rate of the Sisource gas may be 4 sccm. The flow rate of the C source gas in the mixedgas may be not less than 0.3 sccm and not more than 1.6 sccm, forexample. The lower limit of the flow rate of the C source gas may be 0.5sccm or 0.7 sccm. The upper limit of the C source gas may be 1.4 sccm or1.2 sccm.

In the step of reconstructing the surface, it is desirable to adjustvarious conditions such that etching by the hydrogen gas is comparableto epitaxial growth by the source material gas. For example, it isconsidered to adjust the flow rate of the hydrogen gas and the flow rateof the source material gas to attain a film formation rate of about0±0.5 μm/h. The film formation rate may be adjusted to about 0±0.4 μm/h,may be adjusted to about 0±0.3 μm/h, may be adjusted to about 0±0.2μm/h, or may be adjusted to about 0±0.1 μm/h. Accordingly, it isexpected to increase the effect of attaining a shallow pit portion.

A treatment time in the step of reconstructing the surface is about notless than 30 minutes and not more than 10 hours, for example. Thetreatment time may be not more than 8 hours, may be not more than 6hours, may be not more than 4 hours, or may be not more than 2 hours.

3. Step (S23) of Epitaxially Growing Second Film

After reconstructing the surface of the first film, the step ofepitaxially growing the second film on this surface is performed. Secondfilm 122 (see FIG. 1) is formed using a source material gas having aC/Si ratio of not less than 1. For example, the C/Si ratio may be notless than 1.05, may be not less than 1.1, may be not less than 1.2, maybe not less than 1.3, or may be not less than 1.4 as long as the C/Siratio is not less than 1. Moreover, the C/Si ratio may be not more than2.0, may be not more than 1.8, or may be not more than 1.6.

The source material gas in the step of epitaxially growing the secondfilm may be the same as or different from the source material gas usedin the step of epitaxially growing the first film. The source materialgas may be silane gas and propane gas, for example. The flow rate of thesilane gas and the flow rate of the propane gas may be adjustedappropriately in a range of about 10 to 100 sccm to achieve a desiredC/Si ratio, for example. The flow rate of the carrier gas may be about50 slm to 200 slm, for example.

The film formation rate in the step of epitaxially growing the secondfilm may be about not less than 5 μm/h and not more than 100 μm/h, forexample. The second film has a thickness of not less than 1 μm and notmore than 150 μm, for example. Moreover, the thickness of the secondfilm may be not less than 5 μm, may be not less than 10 μm, and may benot less than 15 μm. Moreover, the thickness of the second film may benot more than 100 μm, may be not more than 75 μm, or may be not morethan 50 μm.

The thickness of second film 122 may be the same as or different fromthe thickness of first film 121. Second film 122 may be thinner thanfirst film 121. For example, the ratio of the thickness of second film122 to the thickness of first film 121 may be about not less than 0.01and not more than 0.9. Here, the ratio of the thicknesses represents avalue obtained by dividing the thickness of the second film by thethickness of the first film having been through the step ofreconstructing the surface. The ratio of the thicknesses may be not morethan 0.8, may be not more than 0.7, may be not more than 0.6, may be notmore than 0.5, may be not more than 0.4, may be not more than 0.3, maybe not more than 0.2, or may be not more than 0.1. Accordingly, it isexpected to increase the effect of attaining a shallow pit portion.

As described above, as shown in FIG. 1, silicon carbide film 120including first film 121 and second film 122 is formed. In siliconcarbide film 120, the first film and the second film may be incorporatedcompletely such that they cannot be distinguished from each other.

By sequentially performing the step (S10) to the step (S23) as describedabove, epitaxial wafer 100 can be manufactured in which groove portion20 is formed in the surface of silicon carbide film 120.

[Evaluation]

1. Production of Sample

Silicon carbide substrates 110 each having a diameter of 150 mm wereprepared. In each of silicon carbide substrates 110, the off directionof second main surface 102 was the <11-20> direction and second mainsurface 102 had an off angle of 4° relative to the (0001) plane.

A sample 1 had a silicon carbide film formed using the manufacturingmethod according to the present disclosure. A sample 2 had a siliconcarbide film formed using a manufacturing method obtained by omitting,from the manufacturing method according to the present disclosure, thestep (S22) of reconstructing the surface of the first film. In each ofsample 1 and sample 2, the silicon carbide film had a film thickness of15 μm.

2. Evaluation of Shape of Groove Portion

In each sample, the shape of the groove portion formed in first mainsurface 101 was evaluated using a defect inspection device and an AFM.The result is shown in Table 1. The device for inspecting positions ofdefects as used herein was WASAVI series “SICA 6×” (objective lens: ×10)provided by Lasertec Corporation.

The AFM as used herein may be “Dimension 300” provided by Veeco, forexample. Moreover, for a cantilever (probe) of the AFM, “NCHV-10V”provided by Bruker may be used, for example. For measurement conditionsof the AFM, a measurement mode was set at a tapping mode, a measurementarea in the tapping mode was set at a square having each side of 20 μm,and a measurement depth was set at 1.0 μm. Moreover, sampling in thetapping mode was performed under conditions that scanning speed in themeasurement area was set at 5 seconds for one cycle, the number of datafor each scan line was set at 512 points, and the number of the scanlines was set at 512. Moreover, displacement control for the cantileverwas set at 15.50 nm.

TABLE 1 Sample 1 Sample 2 Maximum Depth (Second Depth) 3 nm 25 nm ofFirst Groove Portion Depth (First Depth) of Second Not More Than 1 nm —Groove Portion Width (Second Width) in One 25 μm 1 μm Direction Width(Third Width) in Perpendicular 2 μm 1 μm Direction

As shown in Table 1, in sample 1, groove portion 20 was detected inwhich second width 82 was twice or more as large as third width 83.Second width 82 is a width that extends in the step-flow growthdirection (i.e., “one direction”) along first main surface 101 and thatis in the step-flow growth direction, and third width 83 is a width thatis in the direction perpendicular to the step-flow growth direction.

Further, as a result of detailed inspection on the shape of grooveportion 20 in sample 1, it was found that a portion exhibiting themaximum depth was included in one end portion within groove portion 20.The depth of the portion exhibiting the maximum depth was 3 nm. Thedepth of a portion extending from this portion to the other end portionwas not more than 1 nm. That is, groove portion 20 in sample 1 includedfirst groove portion 21 and second groove portion 22 connected to firstgroove portion 21, wherein first groove portion 21 was formed at one endportion of groove portion 20 in the step-flow growth direction, secondgroove portion 22 extends in the step-flow growth direction from firstgroove portion 21 to the other end portion opposite to the one endportion, and first depth 71, which was a depth from first main surface101, was smaller than second depth 72, which was the maximum depth ofthe first groove portion.

On the other hand, in sample 2, a multiplicity of groove portions, i.e.,pit portions 30, were detected in each of which second width 82 andthird width 83 were substantially the same and second depth 72, i.e.,the maximum depth was more than 10 nm. In Table 1, for convenience, themaximum depth of the groove portion in sample 2 is illustrated in thecolumn for the maximum depth of the first groove portion.

3. Evaluation of Variation in Film Thickness of Oxide Film

By heating samples 1 and 2 in an atmosphere including oxygen, an oxidefilm was formed on first main surface 101 of silicon carbide film 120.Furthermore, the oxide film was observed with a transmission electronmicroscope to measure variation in film thickness of the oxide film. Theresult is shown in Table 2.

TABLE 2 Sample 1 Sample 2 Film Thickness of Portion Having No 52 52Groove Portion (nm) Minimum Film Thickness in the 51 49 Vicinity ofGroove Portion (nm) Maximum Film Thickness in the 51 60 vicinity ofGroove Portion (nm) Variation in Film Thickness (A/B) −1/−1 +8/−3

In the column “Variation in Film Thickness” in Table 2, “A/B” isillustrated to represent a difference (A) between the maximum filmthickness in the vicinity of the groove portion and the film thicknessof the portion having no groove portion, as well as a difference (B)between the minimum film thickness in the vicinity of the groove portionand the film thickness of the portion having no groove portion. Here, itis indicated that as A and B are both smaller values, variation in filmthickness is smaller. As shown in Table 2, the variation in filmthickness in sample 1 was smaller than that in sample 2 and thereforesample 1 was excellent.

The embodiments disclosed herein are illustrative and non-restrictive inany respect. The scope of the present invention is defined by the termsof the claims, rather than the embodiments described above, and isintended to include any modifications within the scope and meaningequivalent to the terms of the claims.

REFERENCE SIGNS LIST

1: epitaxial growth device; 2: channel; 3: induction heating coil; 4:quartz tube; 5: heat insulator; 6: heating element; 7: curved portion;8: flat portion; 100: epitaxial wafer; 101: first main surface; 102:second main surface; 103: third main surface; 110: silicon carbidesubstrate; 120: silicon carbide film; 121: first film; 122: second film;20: groove portion; 21: first groove portion; 22: second groove portion;30: pit portion; 40: threading dislocation; 71: first depth; 72: seconddepth; 81: first width; 82: second width; 83: third width; 84: fourthwidth.

The invention claimed is:
 1. An epitaxial wafer comprising: a siliconcarbide film having a first main surface; a silicon carbide substratehaving a second main surface, the silicon carbide film being formed onthe second main surface, the silicon carbide film having a thickness ofnot less than 5 μm, the second main surface having an off angle of notmore than ±4° relative to a (0001) plane, an off direction of the offangle being in a range of not more than ±5° relative to a <11-20>direction or in a range of not more than ±5° relative to a <01-10>direction, a groove portion being formed in the first main surface, thegroove portion extending in the off direction along the first mainsurface, a width of the groove portion in the off direction being twiceor more as large as a width of the groove portion in a directionperpendicular to the off direction, the maximum depth of the entirety ofthe groove portion from the first main surface being not more than 10nm, wherein the groove portion includes a first groove portion and asecond groove portion connected to the first groove portion, the firstgroove portion is formed in one end portion of the groove portion in theoff direction, the second groove portion extends in the off directionfrom the first groove portion to the other end portion opposite to theone end portion, and a depth of the second groove portion from the firstmain surface is smaller than the maximum depth of the entirety of thefirst groove portion, the first groove portion has a triangular shape incross-section, and the second groove portion comprises a bottom surfacethat is substantially parallel to the first main surface.
 2. Theepitaxial wafer according to claim 1, wherein the groove portion extendsin the off direction of the off angle from a threading dislocation inthe silicon carbide film.
 3. The epitaxial wafer according to claim 1,wherein the groove portion is formed due to at least a threading screwdislocation.
 4. The epitaxial wafer according to claim 1, wherein thewidth of the groove portion in the off direction is not less than 15 μmand not more than 50 μm.
 5. The epitaxial wafer according to claim 1,wherein the first groove portion comprises a first side wall and asecond side wall that form the triangular shape in cross-section, thefirst side wall extending from the first main surface to the second sidewall, and the second side wall extending from the first side wall towardthe first main surface to a depth in the silicon carbide film below thefirst main surface.